138.

Every shadow cast by a body has a central line directed to a single
point produced by the intersection of luminous lines in the middle
of the opening and thickness of the window. The proposition stated
above, is plainly seen by experiment. Thus if you draw a place with
a window looking northwards, and let this be s f, you will see a
line starting from the horizon to the east, which, touching the 2
angles of the window o f, reaches d; and from the horizon on the
west another line, touching the other 2 angles r s, and ending at
c; and their intersection falls exactly in the middle of the
opening and thickness of the window. Again, you can still better
confirm this proof by placing two sticks, as shown at g h; and you
will see the line drawn from the centre of the shadow directed to
the centre m and prolonged to the horizon n f.